A number of food ingredients and foodstuffs have been produced from the hydrolysis of a protein source such as the milk proteins, casein and whey proteins.
Hydrolysed protein foodstuffs may have advantages over non-hydrolysed protein foodstuffs in a number of areas of health care. For example, it is known that enzymatically hydrolysed proteins are less allergenic. They are also more rapidly digested and absorbed than whole proteins. Foodstuffs containing hydrolysed proteins are also useful in the alimentation of hospital patients with digestive diseases for example.
Hydrolysis of whey proteins and caseins is known to release bioactive peptides that can exhibit a number of physiological effects (Maubois et al, 1991; EP 475506). A number of publications describe such bioactive peptides, for example, ACE inhibiting peptides which have antihypertensive properties have been released through an enzymatic treatment of bovine β-lactoglobulin and whey protein concentrates (Mullally et al, 1997). ACE inhibitory peptides are also found in sour milk and in hydrolysates of α, and β casein (JP 4282400; Nakamura et al 1994, Yamamoto et al 1994).
EP 4745506 discloses the hydrolysis of the milk protein lactoferrin in whey to release lactoferricin which acts as an antimicrobial agent useful for treating diarrhoea, athlete's foot, eye infections, mastitis etc in humans and animals.
However, the hydrolysis of most food proteins, especially the hydrolysis of whey and casein containing products, is known to generate bitterness. This causes palatability problems particularly when attempting to formulate orally ingestible products incorporating milk protein hydrolysates as a source of bioactive peptides.
In the field of protein hydrolysis one or both of two approaches are commonly used for controlling or removing bitterness in protein hydrolysates to increase palatability of the products.
The extensive hydrolysis of the protein substrate is known to reduce bitterness in milk protein hydrolysates (EP 065663; EP 117047; U.S. Pat. No. 3,970,520). Less bitter products are produced relatively easily and cheaply in this way. However, extensive hydrolysis reduces the chain lengths of all peptides, including the bioactive peptides of interest. Extensive hydrolysis of the protein substrate destroys the functional and biological activity of the peptide of interest. In addition soapy and brothy off-flavours often develop, with the consequence that the palatability of the final product remains poor compared to the original bland tasting protein substrate. A final disadvantage is that for some hydrolysates the bitterness is only partially removed (Roy 1992 and 1997).
A second common method for the control of bitterness in protein hydrolysates is to use debittering enzymes, in particular those sourced from Aspergillus oryzae. 
“Bitterness” generation in protein hydrolysis is thought to be due to the presence of large hydrophobic ‘bitter’ peptides. Debittering enzymes selectively hydrolyse bitter peptides present in the protein hydrolysates. A worker skilled in the art can—by the judicious selection of debittering enzymes and the conditions of treatment—effectively debitter milk protein hydrolysates leaving intact the particular bioactive peptides of interest. However, use of debittering enzymes makes the process more expensive, and preservation of some of the bioactive peptide is not easily or sucessfully achieved. A further disadvantage is that debittering enzymes treatments have a tendency to release free amino acids into the final product and, as a consequence, the hydrolysates develop unpleasant brothy or soapy flavours (Roy 1992 and 1997).
The various methods of debittering the protein hydrolysates result in additional process steps and add costs to the manufacture of the final product. In addition the final product also becomes overbalanced in its supply of free amino acids.
It would be most advantageous if a process for hydrolysing protein could be developed which releases bioactive peptides of interest and which limits the formation of bitter peptides and free amino acids, thereby allowing the original bland taste of the milk proteins substrates to be retained.
Some bioactive peptides—in particular the antihypertensive peptides—are relatively stable during protein hydrolysis and are released very early during the hydrolysis of the milk protein substrate as shown in FIG. 1.
The bitter flavours of milk protein hydrolysates can be improved by adding sugars or by hydrolysing natural sugars, such as lactose, already present in the milk protein substrate (Bernal and Jelen, 1989). For example sour wheys and cheese wheys are made more palatable when they have been sweetened by β-galactosidase and lactose hydrolysis of lactose (FR 2309154; U.S. Pat. No. 4,358,464; JP 8056568).
In order to achieve a high flavour acceptability for a hydrolysed protein product which contains bioactive peptides, precise control of hydrolysis is required to prevent bitterness occurring.
A common method of termination of hydrolysis is by deactivation of the enzymes, usually by thermal deactivation at high temperatures, typically>90-100° C. for an extended period of time. However, this method cannot be used to stop the hydrolysis of whey proteins as any intact hydrolysed whey proteins remaining in the mixture would denature and precipitate making the final product less soluble and less acceptable for the use as a food ingredient.
It would be advantageous if a process of hydrolysing whey protein could be controlled so that it directly pro a hydrolysate comprising bioactive peptides for incorporation into functional foods which did not taste bitter and where the enzyme inactivation steps did not compromise the integrity of the intact proteins in the final product.
It is an object of the invention to go some way towards achieving these desiderata or at least to offer the public a useful choice.